Service brake systems and parking brake systems are two kinds of brake systems widely used in vehicles. A service brake system is a braking system used to slow or stop a moving vehicle, while a parking brake system is used to prevent a stationary vehicle from moving or rolling away. Traditional parking brake systems were manually hand operated and are also called ‘handbrakes’. A driver needed to pull up the handbrake manually to prevent a stationary vehicle from rolling away and needed to manually release the handbrake to allow the vehicle to move.
Electric parking brake (also called EPB) systems were developed to replace the traditional handbrake systems. When a vehicle is stationary, an EPB system will automatically lock the wheels to prevent the vehicle from rolling. When a driver tries to get a stationary vehicle to move, the EPB system will unlock the wheel automatically. The EPB system comprises brake components, such as calipers and brake pads, to frictionally lock the wheels and an electric motor to actuate the calipers. FIG. 5 illustrates a winding scheme of a traditional permanent magnet direct current (PMDC) motor used in an EPB system. The rotor windings comprise a plurality of coils wound about teeth of the rotor core and electrically connected to segments of a commutator. For example, coil C31 is wound about teeth T1 and T2 and connected to segments S1 and S2. Coil C32 is wound about teeth T2 and T3 and connected to segments S2 and S3. The PMDC motor comprises two brushes to feed current to the rotor windings. The rotor windings form two parallel branches. When a coil such as coil C31 is open, one of the branches will be open. The motor as well as the EPB system will malfunction. It is dangerous to drive a car having a malfunctioning EPB system.
Therefore, there is a desire for an EPB system with an improved reliability.